In a nuclear power plant, individual fuel assemblies within the plant's reactor vessel must be accurately positioned during reactor operation. This is accomplished by a multitude of fuel assembly alignment pins which are part of the reactor vessel internals structures. The fuel assembly alignment pins project from upper and lower core plates of the internals structures. There are four alignment pins for each fuel assembly: two which extend from the upper core plate and two from the lower core plate. These pins are received in corresponding holes within the fuel assembly top and bottom nozzles. These pins also provide a structural support for the fuel assemblies within the reactor vessel. Therefore, fuel assembly alignment pins are an important component of the nuclear reactor internals structures.
Over the years a number of these pins have become bent at various power plants during reactor refueling operations. To remove spent fuel assemblies, the reactor upper internals are removed from the reactor vessel. During refueling, fresh fuel assemblies are loaded into the lower internals structure and then the upper internals are replaced atop the fuel assemblies. This repeated removal and replacement of the internals structures and fuel assemblies has resulted in the inadvertent bending of several fuel assembly alignment pins. Since a main purpose of these pins is to provide precise alignment of fuel assemblies within the reactor core for proper insertion of control rods, for example, a misaligned pin which cannot be received within the top or bottom nozzle of a fuel assembly can lead to extended fuel reload outages.
A solution to this problem has been to remove such bent pins from the upper and lower core plates of the internals prior to refueling and startup of the reactor. Not only is considerable time lost in removing such a bent pin, but also a large analytic and licensing effort is required to justify operation of the reactor without the alignment pin. This is also due in part to the fact that alignment pins provide structural support for fuel assemblies. Such procedures can result in a high cost for the nuclear reactor power plant operators. Whereas the removal of only a relatively small number of pins may be acceptable, the removal of many such pins cannot be justified. What is needed then, is a way to bring a bent pin back into proper alignment. And since these are irradiated components, such procedures should be capable of remote operation so as to minimize man-rem exposure.
There exists in the prior art various methods and apparatuses for straightening an array of terminal pins or leads of electronic connectors or circuit boards, examples of which are U.S Pat. Nos. 3,700,011 to Walter, 4,397,341 to Kent, and 4,340,092 to Chisholm. The basic method taught by each of these patents is to bend or twist the pins in a reciprocating manner in orthogonal directions, beyond the pins' elastic limit, in order to straighten an array of terminal pins. Since these pins are very small, having a usually square cross-section of only 0.635 mm (0.025 inch), and are made of a soft resilient material, e.g. half-hard brass or copper, it is very easy to straighten terminal pins by such methods. Another characteristic of these prior art methods and apparatuses is that as many terminal pins are straightened simultaneously as possible. Typically, as many as 10,000 terminal pins are inserted in a single circuit board, on a grid spacing of 3.175 mm (0.125 inch). Thus, such methods do not require a very large device to simultaneously straighten numerous terminal pins. Furthermore, due to such close spacing, it is very difficult to individually detect and straighten a single misaligned pin. Therefore, since terminal pins are resilient, it is far easier to simultaneously manipulate all such pins whether or not they require straightening, than it is to seek out and straighten only those pins which are actually bent.
Whereas such methods and apparatuses are quite capable of correctly aligning circuit board terminal pins, they are not suitable for straightening bent fuel assembly alignment pins. By way of example only, reference is made to a four-loop Westinghouse pressurized water reactor. Firstly, one design of such a pressurized water reactor has an internal diameter, having corresponding upper and lower core support plates on which the fuel assembly alignment pins are located, of about 3000 mm (10 feet). Secondly, there are on the order of 200 fuel assemblies in such a reactor vessel, resulting in about 400 alignment pins in each of the upper and lower core support plates. Finally, fuel assembly alignment pins for such a reactor design are typically made of stainless steel having a diameter of about 22 mm (or less than 1 inch), which does not lend itself to easy bending or twisting. Hence, it would not be advantageous to attempt to simultaneously straighten substantially all of the alignment pins, particularly since only a relatively small number of pins, if any, are bent.
It is therefore an object of the present invention to provide a device to accurately straighten bent fuel assembly alignment pins.
It is a further object of the present invention to provide such a device which is capable of remote operation.
It is a still further object of the present invention to provide a device which can accurately straighten bent fuel assembly alignment pins in various nuclear power plants having non-uniform refueling cavities.